This application relates in general to hydrostatic transmissions and in particular to a dual pump arrangement. Hydrostatic pumps are well-known for use in driving vehicles such as tractors and other off-road devices. Such pumps are also used in a wide variety of industrial applications other than vehicles.
In one known arrangement for a vehicle, a plurality of pumps are mounted in separate housings on a vehicle frame. The pumps are each connected to a respective hydrostatic motor through high pressure hoses, which are often connected to end caps. The end cap is secured to the pump housing and includes a running surface for the pump and porting to connect the pump to the hoses.
A control arm is engaged to each hydrostatic pump to control the output of the pump. In a known design, the hydrostatic pump is of an axial piston design and the control arm is engaged to a swash plate, the rotation of which can change the output of the pump from forward to neutral to reverse. Rotation of the pumps is provided by rotary input shafts which are separately driven by the vehicle engine by pulleys and belts or other known methods. The pump transmits hydraulic fluid through one of a pair of high pressure hoses to a hydrostatic motor. Rotational output of the motor is then transmitted to the vehicle drive wheels through an output axle or other known means.
Such an arrangement allows for zero turn capability, since the hydrostatic pumps may be operated independently of one another. However, there is a cost involved with this arrangement, as it requires at least four separate housings for the individual pumps and motors, and each housing must be individually secured to the vehicle frame.
Another known hydrostatic transmission is the BDU transmission. This hydrostatic transmission comprises a single housing enclosing both a hydrostatic pump and a hydrostatic motor, both of which are mounted to a single plate. The pump input shaft and motor output shaft are parallel to one another, and the plate contains hydraulic porting to connect the pump and motor. One such hydrostatic transmission is shown in U.S. Pat. No. 5,392,670. Such an HST is generally used to connect to a drive train for powering output axles of a tractor or similar vehicle.
It is an object of this invention to provide a lower cost hydrostatic pump design that can be used in, e.g., a zero turn vehicle, or in industrial applications. This invention in the preferred embodiment uses a dual pump design having two pumps mounted in a side-by-side arrangement within a single housing. The housing can include an end cap or plate having hydraulic porting therein. High pressure hoses engaged to the end cap or plate are then connected to a plurality of hydrostatic motors engaged to vehicle drive wheels. Control arms can, be mounted on various locations on the common housing to independently control the pumps.
A benefit of this design is that it eliminates the need for separate housings for the two pumps, and reduces the number of mounting points required on the vehicle. A further advantage is that it eliminates the need for separate drive inputs for the two pumps. In the preferred embodiment, a single input shaft drives both pumps. This input shaft could directly drive one pump and be engaged to and drive a second input shaft for the second pump through gearing which could be either internal or external to the common pump housing. A benefit of this arrangement is reduced cost and size, and the ability to maintain both pumps at a constant speed.
A key feature of this design is the flexibility it affords to the user of the apparatus. For example, with minimal design changes, one or more charge pumps and/or auxiliary pumps may be attached to the primary input shaft or the secondary input shaft as needed to provide charge fluid to the hydraulic circuit or to power additional units, such as mowers, deck lifts, as may be needed.
A further benefit of this design is the ability to use an input shaft having a varying diameter, which permits the use of, among other things, a more substantial auxiliary pump on this shaft. Another benefit is the location of the two trunnion arms to operate the pump swash plates on opposite sides of the pump housing, corresponding to the sides of the vehicle, to avoid clearance problems and to simplify connection of the trunnions to the control mechanism of the vehicle. Prior art designs have the trunnions extending from one side, corresponding to the front or back of the vehicle, which thus requires additional linkages to turn the controls 90 degrees. A further benefit is the use of a single bypass valve for both pumps and the location of this valve on the side of the housing which is different from the mounting of the two trunnion arms.
While the preferred embodiment uses a single pump cavity inside the housing to mount both of the pumps, it is possible to use separate cavities to independently mount the pumps, which would permit varying pressures to be used for industrial applications and other uses where such different pressures may be important.
Another benefit of this design is the ability to locate a cooling fan on the top of the unit, with the drive input or primary input shaft entering from the bottom of the unit. This design protects the cooling fan from debris that may be kicked up during operation at the bottom of the unit. One could also use a return to neutral mechanism with this design in a known manner, such as that described and shown in co-pending application Ser. No. 09/789,419 entitled xe2x80x9cZero-Turn Transaxle with Mounted Return to Neutral Mechanism,xe2x80x9d the terms of which are incorporated herein by reference.